A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The great majority of today's rigid-frame bicycles have a frame with upright seating. Such upright rigid-frame bicycles generally feature the diamond frame, a truss consisting of two triangles: the front triangle and the rear triangle. In a conventional diamond frame, the “front triangle” is not a true triangle because it consists of four tubes: the head tube, top tube, down tube and seat tube. The head tube contains the headset, the set of bearings that allows the front fork (which supports the front wheel) to turn smoothly for steering and balance. The top tube connects the head tube to the seat tube at the top, and the down tube connects the head tube to the bottom bracket. The rear triangle consists of the seat tube and paired chain stays and paired seat stays. The chain stays run essentially parallel to the chain, connecting the bottom bracket to the rear fork ends (which support the rear wheel). The seat stays connect the top of the seat tube (at or near the same point as the top tube) to the rear fork ends.
Many modern bicycles do not utilize a diamond frame, for example because: the frame is constructed in such a way that it does not consist of tubes attached one to another (for example, frames made of composite materials); or the frame involves a rear suspension system permitting rearward components of the bicycle (e.g., the rear wheel) to move relative to other components of the bicycle (e.g., the seat); or both. However, the terms used to describe the members of a conventional diamond frame (being, head tube, top tube, down tube, seat tube, chain stays and seat stays) are often used to describe analogous features on non-diamond frames and are at times so used herein.
Most bicycles use a chain to transmit power to the rear wheel. The drivetrain begins with pedals which rotate the cranks, which are attached to a spindle that rotates within the bottom bracket. With a chaindrive, a chainring attached to a crank drives the chain, which in turn rotates the rear wheel via a rear sprocket. Most chaindrive systems have some form of gearing, typically comprising multiple rear sprockets of different sizes, multiple chainrings of different sizes and user controllable devices (referred to as derailleurs) for moving the chain between rear sprockets and between the chainrings, so as to selectively vary the gear ratio. In chain drive systems, the portion of chain extending between the top of a chainring and the top of a rear sprocket conveys the motive force from the pedals to the rear wheels. When the rider is pedalling, this top portion of chain is under tension. In a bicycle without a rear suspension, this chain tension is resisted by the the rear triangle, to which the rear wheel is mounted. However, in a bicycle with a rear suspension system, some portion of the force of such chain tension may be imparted to the suspension system. As well, movement of the rear suspension system relative to the bottom bracket may dynamically tension or slacken the portion of chain extending between the top of a chainring and the top of a rear sprocket, thereby affecting the pedalling resistance experienced by the rider. The direction of the force conveyed along the portion of chain extending between the top of a chainring and the top of a rear sprocket is referred to as the chain line. A further complication is that bicycles typically have multiple chainrings and multiple rear sprockets so as to provide rider selectable gear ratios; in the result, most bicycles would not have a single chain line, but rather would have multiple chain lines.
A bicycle suspension is the system or systems used to suspend the rider and all or part of the bicycle in order to protect them from the roughness of the terrain over which they travel. Bicycle suspensions are used primarily on mountain bikes, but are also common on hybrid bicycles, and can even be found on some road bicycles. Bicycle suspension can be implemented in a variety of ways, including: front-fork suspension and rear suspension. It is not uncommon for a mountain bike to have front suspension but no rear suspension (such a suspension configuration is often referred to as a hardtail). However, it is uncommon for a mountain bike to have a rear suspension system but no front suspension system. Thus, rear suspension systems on mountain bikes are typically part of a full suspension system.
Suspension systems for mountain bikes first appeared in roughly the early 1990's. Over the ensuing years developers and users of mountain bike suspension systems recognized a variety of factors affecting suspension performance and general riding performance of suspension system, which factors are interrelated in dynamic and complex ways. It was soon realized that the fact that bicycles are powered by human effort means that effects on the drive train caused by suspension system movement that would, in the case of engine driven vehicles, be minor or unnoticeable, are significant in bicycles. In particular, rear suspension systems involve complicated interactions of multiple connected components and multiple performance considerations.
In the field of bicycle suspension systems, the following terms are generally used as follows:                Travel generally refers to how much movement a suspension allows, and is usually quantified based on the available range of movement of the wheel axle.        Brake jack refers extension of the rear suspension caused by braking (a feature of some early suspension designs).        Brake squat refers to compression of the rear suspension caused by braking (which in moderation, can be beneficial to counteract the normal forward weight transfer caused by braking).        Bob, pedal bob, or monkey motion refer to undesirable repeated compression and rebound with each pedal stroke.        Squat refers to generally undesirable compression of the rear suspension under acceleration (and the associated rearward weight shift).        Pedal feedback (or chainstay lengthening) refers to torque applied to the crankset by the chain caused by motion of the rear axle relative to the bottom bracket. Pedal feedback is caused by an increase in the distance between the chainring and rear sprocket, and it can be felt by the rider as a torque on the crankset in the rotational direction opposite to forward pedalling.        Anti-squat refers to chainstay lengthening related to pedalling-induced suspension extension, which provides resistance to the weight shift of the rider due to acceleration and resulting compression of the rear suspension. Too much anti-squat or chainstay lengthening results in resistance to compression of the suspension due to pedal forces when the rear wheel hits an obstacle.        Preload refers to the force applied to spring component before external loads, such as rider weight, are applied. The amount of preload necessary depends on the rider weight and the parameters of the spring components. More preload makes the suspension sag less and less preload makes the suspension sag more. Adjusting preload affects the ride height of the suspension.        Rebound refers to the rate at which a suspension component returns to its original configuration after absorbing a shock. The term also generally refers to rebound damping or rebound damping adjustments on shocks, which vary the rebound speed. Increasing rebound damping causes the shock to return at a slower rate.        Sag refers to how much a suspension moves under just the static load of the rider. Sag allows the rear wheel to drop into depressions in the terrain, maintaining traction.        Sag point refers to a design/tuning parameter, being a desired suspension sag for a rider, which is generally between 20-35% of the total suspension travel depending on the rider's preference and the suspension design.        Compression damping refers to systems that slow the rate of compression in a front fork shock or rear shock. Compression damping is usually accomplished by forcing a hydraulic fluid (such as oil) through a valve when the shock becomes loaded and is often adjustable.        Unsprung mass is the mass of the portions of bicycles that is not supported by the suspension systems.        
One of the simplest and most common bicycle suspension designs is the single-pivot system, in which the rear wheel of the bicycle is attached to the front triangle of the bicycle by a single swingarm (often a generally triangular component and often referred to as the rear triangle) pivoting about a pivot located on the front triangle. With the single-pivot design, the rear wheel absorbs bumps from irregular terrain by moving in a simple curve (i.e., a circular arc) about the pivot.
More complicated suspension designs use a configuration of linkages that is more complicated than a mere single pivot and that generally provide for an axle path of travel during suspension compression and extension that is other than the simple curve about the pivot point achievable with the single-pivot suspensions. A popular linkage suspension design is shown in FIG. 3 in U.S. Pat. No. 5,899,480 (commonly referred to as a Horst Link suspension system after the inventor, Horst Leitner). Dual short-link designs are a popular type of four-bar linkage suspension systems comprising two short links interposed between the front triangle and the rear triangle (i.e. the component to which the rear wheel is mounted). A dual short link design called the Virtual Pivot Point suspension (or VPP), is disclosed in U.S. Pat. No. 6,206,397. A dual short link design that employs links pivoting in the same direction is disclosed in U.S. Pat. No. 7,128,329 (Weagle).
Many of the patented dual short link suspension designs featuring two short links rotating in the same direction emulate the function of Weagle's or the VPP designs in various ways, but differ with respect to the placement, length and pivot locations of the two short links. The chainstay lengthening/anti-squat effects are derived from the placement of the links and pivot points. Many known designs focus on the designer's version of optimal anti-squat characteristics, minimizing overall chainstay lengthening to varying degrees, the use of low speed compression damping on the shock absorber to reduce unwanted suspension movement, and minimizing the effects of the rear brake on the suspension system.
Many known suspension designs endeavour to optimize pedalling efficiency by providing sufficient anti-squat to balance the rearward weight shift due to acceleration, in selected optimal gear combinations, which balancing is referred to as 100% anti-squat. The value of anti-squat depends on acceleration. If there is no acceleration, anti-squat is irrelevant. The greatest rate of acceleration of a bicycle is achieved when accelerating from a standstill or from a low speed, with the rate of acceleration (and the amount of anti-squat required to balance squat) quickly dropping off as one approaches the desired speed of travel.
Numerous bicycle systems and variations of same are known. For example, as described in the following US patents: U.S. Pat. No. 5,553,881, BICYCLE REAR SUSPENSION SYSTEM, Klassen et al., 10 Sep. 1996; U.S. Pat. No. 5,628,524, BICYCLE WHEEL TRAVEL PATH FOR SELECTIVELY APPLYING CHAINSTAY LENGTHENING EFFECT AND APPARATUS FOR PROVIDING SAME, Klassen et al., 13 May 1997; U.S. Pat. No. 6,206,397, BICYCLE WHEEL TRAVEL PATH FOR SELECTIVELY APPLYING CHAINSTAY LENGTHENING EFFECT AND APPARATUS FOR PROVIDING SAME, Klassen et al., 27 Mar. 2001; U.S. Pat. No. 6,843,494, REAR SUSPENSION SYSTEM FOR TWO-WHEELED VEHICLES, PARTICULARLY BICYCLES, Lam, 18 Jan. 2005; U.S. Pat. No. 6,969,081, BICYCLE REAR SUSPENSION, Whyte, 29 Nov. 2005; U.S. Pat. No. 7,128,329, VEHICLE SUSPENSION SYSTEMS, Weagle, 31 Oct. 2006; U.S. Pat. No. 7,240,912, BICYCLE REAR SUSPENSION, Whyte, 10 Jul. 2007; U.S. Pat. No. 7,828,314, VEHICLE SUSPENSION SYSTEMS, Weagle, 9 Nov. 2010; U.S. Pat. No. 7,934,739, BICYCLE REAR SUSPENSION, Domahidy, 3 May 2011; US 2008/0,054,595 BICYCLE FRAME WITH A COUNTER-ROTATING FOUR BAR LINKAGE SYSTEM, Lu, 6 Mar. 2008; US 2008/0,277,900, BICYCLE WITH A COMMON PIVOT SHOCK ABSORBER, I, 13 Nov. 2008; U.S. Pat. No. 7,048,292, BICYCLE SUSPENSION SYSTEMS, Weagle, 23 May 2006; and US 2014/0,042,726, SUSPENSION SYSTEM FOR WHEELED VEHICLES, Canfield et al., 13 Feb. 2014.